Effect of oxygen isotope substitution on the magnetic structure of $({\mathrm{La}}_{0.25}{\mathrm{Pr}}_{0.75}{)}_{0.7}{\mathrm{Ca}}_{0.3}{\mathrm{MnO}}_3$

The oxygen isotope effect on the magnetic structure and charge ordering in $({\mathrm{La}}_{0.25}{\mathrm{Pr}}_{0.75}{)}_{0.7}{\mathrm{Ca}}_{0.3}{\mathrm{MnO}}_3$ was studied by means of neutron powder diffraction. At first it was found that the two investigated samples, one containing the natural mixture of isotopes (99.7% ${}^{16}\mathrm{O}$, metallic at $T<~100$ K), and the other one enriched by ${}^{18}\mathrm{O}$ in 75% (insulating in the whole temperature range), are identical at room temperature. At the temperature lowering the sample with ${}^{16}\mathrm{O}$ undergoes subsequent antiferromagnetic ${(T}_{\mathrm{AFM}}=150\mathrm{}\hspace{0.5em}\mathrm{K})$ and ferromagnetic ${(T}_{\mathrm{FM}}=110\mathrm{}\hspace{0.5em}\mathrm{K})$ transitions, while in the sample with ${}^{18}\mathrm{O}$ pure AFM ordering ${(T}_{\mathrm{AFM}}=150\mathrm{}\hspace{0.5em}\mathrm{K})$ is found. The temperature dependencies of the neutron diffraction peak intensities associated with charge ordering are also quite different in the samples with ${}^{16}\mathrm{O}$ and ${}^{18}\mathrm{O}$ and correlate with the behavior of the electrical resistivity and the magnetic structure. The temperature behavior of the intensities of the CO and AFM peaks may indicate the presence of phase segregation of the sample with O-16 onto AFM-insulating and FM-metallic phases at low temperature, though no indications of a macroscopic phase segregation were found.